Adhesive Strips For Assembly, Especially Formed with Three Layers and Based on Thermally Cross-Linked, Viscoelastic Acrylate Hot-Melt Adhesives

ABSTRACT

The invention describes a double-sided pressure-sensitive adhesive tape of the kind which can be used in particular for achieving very durable bonds at high temperatures. These adhesive tapes, especially adhesive assembly tapes for the permanent fixing of articles to substrates, are preferably of multilayer construction. The inventive double-sided pressure-sensitive adhesive tapes are characterized in that the viscoelastic carrier layer is composed of a photoinitiator-free, thermally homogeneously crosslinked acrylate hotmelt. They are further characterized in that these homogeneously thermally crosslinked acrylate hotmelts have crosslinking sites by way of urethane units. This viscoelastic carrier layer is covered on both sides in each case by adhesive layers, preferably pressure-sensitive adhesive layers. These layers are connected by chemical reaction to the viscoelastic carrier layer. The operation for producing the adhesive tapes constitutes the corona pretreatment of the adhesive layers following their coating onto release material, the two-dimensional application and shaping of the acrylate hotmelt for thermal crosslinking, a thermal crosslinker having been added to the hotmelt prior to coating, the lamination of the layers together, and the afterreaction on the product in bale form, preferably without introduction of additional heat for crosslinking. This method ensures particularly effective anchoring even of different materials, which is particularly positive for the technical adhesive properties of the adhesive assembly tape.

This application is a division of application Ser. No. 11/572,959, filedOct. 3, 2007, now pending, which is a 371 of PCT/EP2005/054508 filed 9Sep. 2005 (international filing date) claiming priority of GermanApplication DE 10 2004 044 086.7, filed Sep. 9, 2004.

The invention describes a double-sided pressure-sensitive adhesive (PSA)tape, in particular a PSA tape of three-layer construction, of the kindwhich can be used in particular for achieving very durable bonding athigh temperatures, and also a method of producing it.

BACKGROUND OF THE INVENTION

For industrial PSA tape applications it is very common to employdouble-sided PSA tapes in order to join two materials to one another. Adistinction is made here, depending on type, between single-layerdouble-sided self-adhesive tapes and multilayer double-sidedself-adhesive tapes.

Single-layer double-sided self-adhesive tapes, known as transfer tapes,are constructed such that the PSA layer contains no carrier and is linedonly with corresponding release materials, such as siliconized releasepapers or release films. Transfer tapes may be lined with releasematerials on one side or both sides. Often use is made of release papersor release films with different degrees of siliconization on eitherside, so that the transfer tape can be wound readily into a roll andthen also applied readily. Adhesive transfer tapes are frequently usedin order to provide any of a very wide variety of substrates withpressure-sensitive adhesion. This is accomplished, for example, bylaminating the transfer tape onto the substrate. In that case therelease paper remains as a liner to the PSA layer in the product.

Relatively thin transfer tapes are often produced with self-adhesivecompositions from solution, whereas thicker transfer tapes are producedwith self-adhesive compositions from the melt or by means of what iscalled UV polymerization. In this procedure a prepolymerized syrupcomposed of acrylate monomers is coated between two UV-transparent,antiadhesively coated release films and is crosslinked on the web by UVirradiation. Specifications that may be mentioned by way of exampleinclude

U.S. Pat. No. 4,181,752, EP 084 220 A, EP 202 938 A, EP 277 426 A, andU.S. Pat. No. 4,330,590. A disadvantage of this technology is the oftenhigh residual monomer fraction in the self-adhesive compositions. Thisfraction of residual monomer is unacceptable for many applications.Transfer tapes filled with non-UV-transparent adjuvants cannot beproduced in this way.

DE 43 03 183 A1 also describes a method of producing thick PSA layers,especially for producing high-performance self-adhesive articles. Insaid process a mixture of starting monomers which is to be polymerizedby means of UV radiation is mixed, and thickened in the process, with asolvent-free, saturated photopolymerizable polymer, and then thismixture is applied to a dehesively treated carrier and exposed to UVradiation. A disadvantage is the use of copolymerized or addedphotoinitiators, since the layers may undergo yellowing and, in theevent of UV exposure prior to use, an often marked change in thetechnical adhesive properties is found. In that case it is necessary togo to considerable effort and expense—by means, for example, ofUV-impervious packaging—to ensure that the customer obtains a uniformlyhigh bonding performance. Moreover, in the event of bonding onUV-transparent substrates, such as on window glass or transparentplastic surfaces, for example, there is a risk that layers containingphotoinitiator will undergo aftercrosslinking. This does resultinitially in an increase in bond strength, but further crosslinkingcauses the layers to become paintlike and undergo embrittlement. Sooneror later, this leads to the failure of the bond, particularly under ashearing load.

Transfer tapes may be foamed or filled in order to improve theirproperties, particularly for example in respect of bonding to unevensubstrates. DE 40 29 896 A1 describes a carrierless, double-sidedself-adhesive tape comprising a pressure-sensitive adhesive layer morethan 200 μm thick which contains solid glass microballs of more than 1.5g/cm³ in density. This tape is said to exhibit particularly effectiveadhesion. A disadvantage is the high density as a result of the glassballs that are used.

Double-sided adhesive tapes of multilayer construction have advantagesover their single-layer counterparts, since the variation of theindividual layers allows specific properties to be set. For instance, athree-layer adhesive tape, consisting of a middle carrier layer and twoouter layers, can be constructed symmetrically or asymmetrically. Thetwo outer layers may each be PSA layers, or, for example, one layer maybe a PSA layer and the other layer a heat-activatable adhesive. Thecarrier, i.e., the middle layer, may for example be a film, a nonwoven,a “non-woven” material or a foam film carrier. Foam or foam likecarriers are often used when there is a requirement for high bondstrength to uneven surfaces or when distances are to be compensated.

For instance, for adhesive assembly tapes, use is often made ofclosed-celled foam carriers based on PE (polyethylene), PU(polyurethane) or EVA (ethyl-vinyl acetate), which have a double-facedcoating of synthetic rubber PSA or acrylate PSA. Applications, listed byway of example, are the bonding of mirrors, trim strips and emblems inautomotive construction, further uses in automobile construction, andalso use in the furniture industry or in household appliances.

Assembly tapes for the exterior sector generally possess PSAs based onpolyacrylate. This material is particularly weathering-resistant andvery long-lived, and is virtually inert toward UV light and towarddegradation by oxidation or ozonolysis.

Also known are adhesive assembly tapes with middle layers of rubber,styrene block copolymers, and polyurethane. All of these materials failto possess the same good aging and thermal stability properties ofpolyacrylate. Systems based on acrylate block copolymers are resistantto aging but are not sufficiently heat-resistant for high-performancerequirements, since these systems are crosslinked only physically by wayof styrene or methyl methacrylate domains. When the softeningtemperature of the domains is reached (as in the case of styrene blockcopolymers), the PSAs soften. Consequently, the bond fails.

Another disadvantage of typical foam adhesive tapes is that they caneasily split. If, for example, PE foam is used, this material softens onheating to about 100° C., and the bond fails. Double-sided assemblytapes of this kind are unsuitable for high-grade applications. Foamsbased on PU are indeed more temperature-stable, but have a tendency toyellow under UV and sunlight exposure. They too are often unsuitable forhigh-performance applications.

For a number of years there have been double-sided adhesive tapesavailable which are of three-layer construction with an acrylate core.This viscoelastic acrylate core is foamlike. Its foamlike structure isachieved through the admixture of hollow glass or polymer balls to theacrylate composition, or else the acrylate composition is foamed bymeans of expandable polymeric “microballoons”. Provided adjacent to thisviscoelastic layer are in each case PSAs, based in the majority of caseslikewise on acrylate, rarely on synthetic rubber, or else in specialcases on heat-activatable adhesive layers. The advantages of theviscoelastic acrylate core arise on the one hand from the physicalproperties of the polyacrylate (which, as already mentioned, are aparticular weathering stability and long life, and substantially inertbehavior toward UV light and toward degradation by oxidation orozonolysis). As a result of the design of the acrylate core layer,determined for example by the comonomer composition, nature andproportion of certain fillers, and the degree of crosslinking, theseproducts are especially suitable for bonding articles to substrateshaving uneven surfaces. Depending on the choice of PSA, a broad spectrumof properties and bond strengths can be covered.

Nevertheless, as a result of their preparation, the aforementionedsystems have critical disadvantages. The viscoelastic acrylate corelayer is prepared by a process of two-stage UV polymerization. In thefirst step of that process a mixture based on acrylate monomers, 10% byweight acrylic acid and 90% by weight isooctyl acrylate for example, isprepolymerized to a conversion of approximately 10%-20% by UVirradiation in a reactor in the presence of a photoinitiator.Alternatively, this “acrylic syrup” can also be obtained by thermallyinitiated free radical polymerization. In the second step this acrylicsyrup, often after further photoinitiator, fillers, hollow glass balls,and crosslinker have been added, is coated between antiadhesively coatedUV-transparent films, and is polymerized to a higher degree ofconversion on the web, by means of repeated UV irradiation, and in thecourse of this polymerization it is crosslinked. The completedthree-layer product is obtained, for example, after the PSA layers havebeen laminated on.

The production of “relatively thick” viscoelastic layers in particularmust in many cases be carried out in the absence of oxygen. In that casethe composition is protected by a lining of film material, and UVinitiation takes place through the films. PE and PP films which aresometimes used for this purpose deform under the conditions ofcrosslinking reaction (in the case of UV-initiated polymerization, heatof reaction is liberated, and can cause deformation ofnon-temperature-resistant film) and are therefore poorly suited.UV-transparent films such as PET are more thermally stable; in thiscase, however, it is necessary to add to the composition aphotoinitiator which reacts to longwave radiation, in order for thereaction to take place. As a consequence of this, these layers have atendency to undergo aftercrosslinking under UV light or sunlight. Thisprocess negates the advantage specific to the polyacrylate as amaterial. A further disadvantage is that fillers not transparent to UVcannot be used. Moreover, as a result of the process, there remains ahigh residual monomer fraction in these products. Possible reduction ofresidual monomer through a reduction in coating speed or throughintensive subsequent drying is not very economic. The maximum achievablelayer thickness is very heavily dependent on the wavelength of thephotoinitiator used. Layers can be produced of up to about 1 mm, albeitwith the disadvantages specified above. Layers any thicker than this arevirtually impossible to obtain.

A particular disadvantage in the case of acrylate layers produced bytwo-stage UV polymerization, UV crosslinking or electron beam treatmentis a more or less strongly pronounced profile of crosslinking throughthe layer. Toward the irradiation source, the UV-crosslinked layer isalways more strongly crosslinked than on the side opposite the UVradiation source. The degree of the crosslinking profile is dependentfor example on the layer thickness, on the wavelength of thephotoinitiator that is used, and also on the wavelength of the radiationemitted by the UV radiation source.

Specifications DE 198 46 902 A1 and DE 101 63 545 A1 propose using EBC(electron beam) irradiation or UV irradiation from both sides in orderto lower the resulting crosslinking profile and to provide virtuallyhomogeneous crosslinking of thick UV-crosslinkable acrylate PSA layersin particular. However, the layers produced in this way still have acrosslinking profile, and, moreover, the process is very costly andinconvenient. Moreover, it would be virtually impossible to use in orderto produce viscoelastic acrylate carriers; instead, the preparation ofPSA layers in particular is described.

A disadvantage of viscoelastic acrylate carriers which exhibit a profileof crosslinking through the layer is their inadequate capacity fordistributing stresses in a uniform way. One side is always eitherovercrosslinked or undercrosslinked. An exact balance can never bestruck between adhesive and cohesive properties for the entire layer,but instead only for a small section.

EBC-crosslinked layers as well always exhibit a profile of crosslinkingin accordance with the layer thickness and the material. WithEBC-crosslinked layers as well it is impossible to set the crosslinkingexactly. Nevertheless, EBC crosslinking proceeds without addedphotoinitiators, thereby removing some, although not all, of thedisadvantages associated with the UV-irradiated layers. Depending on theaccelerator voltage and on the thickness of the material to beirradiated, it is possible to vary the thickness of the irradiatedlayer. Layers above about 500 μm in thickness, particularly if filledwith inorganic fillers such as glass balls, for example, can no longerbe economically irradiated. Here, therefore, there is an upper limit onthe layer thicknesses that are achievable.

SUMMARY OF THE INVENTION

It is an object of the invention, accordingly, to provide an adhesivetape which comprises an acrylate-based viscoelastic carrier layer whichno longer has the abovementioned disadvantages and which instead isnotable for good technical adhesive properties and very good anchoringof the layers to one another and which can be utilized in particular asan adhesive assembly tape. The adhesive tape ought to possess consistentproperties through the carrier layer; in other words, in particular, itought not to exhibit any profile of crosslinking.

DETAILED DESCRIPTION

In accordance with the invention this object is achieved by means of adouble-sided adhesive tape having at least one carrier layer and twoadhesive layers, the carrier layer being composed of aphotoinitiator-free, homogeneously crosslinked polyacrylate, which thusexhibits no profile of crosslinking through the layer.

BRIEF DISCUSSION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of the process for producing theadhesive tape of the invention.

FIG. 2 illustrates the construction of the three-layeradhesive-polyacrylate carrier-adhesive system of the invention in atwo-roll unit.

FIG. 3. illustrates a test of the holding power of the adhesive tape ofthe invention.

The crosslinked, polyacrylate-based carrier layer is in particular acarrier layer which is viscoelastic, i.e., which shows flow behavior ona substrate, and at the same time possesses at least partly elasticproperties.

One preferred embodiment of the invention is given by a three-layeradhesive tape which comprises as its viscoelastic carrier aphotoinitiator-free, homogeneously thermally crosslinked acrylatehotmelt which at least on one side, in particular on both sides, iscovered with a pretreated, preferably corona-pretreated adhesive. Withparticular preference the carrier layer is joined to the adhesive layersby means of chemical reaction.

Between the carrier layer and one or both adhesive layers there may ineach case be a barrier layer, particularly in order to prevent anymigration of additives or chemical compounds.

With different advantageous embodiments of the adhesive tape it is alsopossible for the features of the individual embodiments of the inventionto be combined with one another.

The adhesive tape of the invention, i.e., in particular, the three-layeradhesive assembly tape with a homogeneously crosslinked viscoelasticpolyacrylate carrier, is advantageously obtained by the method describedin the following text: At least one thermal crosslinker is added in themelt, preferably under precise temperature and time control, to apolyacrylate copolymer (referred to simply below as “polyacrylate”)based on acrylic esters and/or methacrylic esters. The polyacrylatetogether with the crosslinker is conveyed to a coating unit, morepreferably with an extruder, more preferably still with a compoundingextruder in which the crosslinker has already been added and in which,where appropriate, the concentration of the polyacrylate has alreadytaken place; in this regard, compare the diagrammatic representation inFIG. 1, where the numbers and symbols have the following definitions:1.1 polyacrylate feed, 1.2: addition of crosslinker, 1.3: extruder, RW:doctor roll; BW: coating roll. In or after this coating unit thematerial is introduced between two adhesive layers each of which hasbeen pretreated, preferably corona-pretreated, and has been coatedtwo-dimensionally onto a carrier material (also referred to as a liner;particularly siliconized release film or release paper). Preference isgiven to coating and lamination by means of double-roll, multiroll ornozzle coating, very preferably with the coating unit illustrated lateron below.

The crosslinking of the polyacrylate introduced in this way takes placein the layer, and in particular the thermal crosslinkers which have beenadded to the polyacrylate react with the pretreated boundary layer. Inthis case there is a chemical attachment of the adhesives to theresultant viscoelastic carrier layer. This provides for effectiveanchoring of carrier material (polyacrylate) and adhesive layers.

The time after the crosslinking system has been metered in thecompounding assembly up until the polyacrylate composition that formsthe carrier is shaped between the adhesives, particularly between theadhesives coated on liners, is referred to as the processing time.Within this time, the viscoelastic carrier layer, which is nowundergoing crosslinking, can be coated gel-free with an optically goodcoating pattern. Crosslinking then takes place primarily after coatingon the web under mild conditions, which are harmful neither to carriernor to liner; in other words, with particular advantage, without theinfluence of actinic radiation such as additional UV irradiation orelectron beams. This produces a homogeneously crosslinked layer, inother words one which does not exhibit a profile of crosslinking throughthe layer.

The base composition used for the carrier layer, in particular for theviscoelastic carrier layer, comprises polyacrylates, which are polymersbased at least partly on acrylic esters and/or methacrylic esters.

With preference in accordance with the invention a portion of theacrylic esters and/or methacrylic esters contains primary hydroxylgroups. In a preferred procedure the fraction of the acrylic and/ormethacrylic esters containing primary hydroxyl groups is up to 25% byweight, based on the polyacrylate.

It may also be of advantage if the polyacrylate includes somecopolymerized acrylic acid.

For the method of the invention for producing the adhesive tape, as abasis for the viscoelastic carrier, it is preferred to use apolyacrylate which can be traced back to the following reactant mixture:

-   -   a) acrylic esters and/or methacrylic esters of the following        formula

CH₂═CH(R^(I))(COOR^(II))

-   -    where R¹═H or CH₃ and R^(II) is an alkyl chain having 1 to 20 C        atoms, with a fraction of 65%-99% by weight,    -   a2) acrylates and/or methacrylates whose alcohol component        contains at least one primary hydroxyl group, and/or vinyl        compounds which are copolymerizable with acrylates and contain        at least one primary hydroxyl group, with a fraction of 1% to        20% by weight,    -   a3) and, if the fractions of a1) and a2) do not add up to 100%        by weight, olefinically unsaturated monomers containing        functional groups, with a fraction of 0% to 15% by weight.

The monomers are preferably chosen such that the resulting polymers havea glass transition temperature of −40° C. to +80° C., understood to be adynamic glass transition temperature for amorphous systems and to be themelting temperature for semicrystalline systems, and being determinableby means of dynamechanical analysis (DMA) at low frequencies.

In order to obtain a correspondingly preferred polymer glass transitiontemperature, T_(g), of −40 to +80° C., and in accordance with the aboveremarks, the monomers are very preferably selected, and the quantitativecomposition of the monomer mixture advantageously chosen, in such a wayas to result in the desired T_(g) value for the polymer in accordancewith an equation (E1) in an analogy to the Fox equation (cf. T. G. Fox,Bull. Am. Phys. Soc. 1 (1956) 123).

$\begin{matrix}{\frac{1}{T_{g}} = {\sum\limits_{n}\; \frac{w_{n}}{T_{g,n}}}} & ({G1})\end{matrix}$

In this equation, n represents the serial number of the monomers used,w_(n) the mass fraction of the respective monomer n (% by weight), andT_(g,n) the respective glass transition temperature of the homopolymerof the respective monomer n, in K.

With great preference use is made for a1) of acrylic or methacrylicmonomers which are composed of acrylic and methacrylic esters havingalkyl groups of 1 to 20 C atoms, preferably 4 to 9 C atoms. Specificexamples, without wishing to be restricted by this recitation, aremethacrylate, methyl methacrylate, ethyl acrylate, n-butyl acrylate,n-butyl methacrylate, n-pentyl acrylate, n-hexyl acrylate, n-heptylacrylate, n-octyl acrylate, n-octyl methacrylate, n-nonyl acrylate,lauryl acrylate, stearyl acrylate, behenyl acrylate, and their branchedisomers, such as isobutyl acrylate, 2-ethylhexyl acrylate, 2-ethylhexylmethacrylate, isooctyl acrylate, and isooctyl methacrylate, for example.Further classes of compound to be used for a1) are monofunctionalacrylates and/or methacrylates of bridged cycloalkyl alcohols, composedof at least 6 C atoms. The cycloalkyl alcohols may also be substituted,as for example by C-1-6 alkyl groups, halogen atoms or cyano groups.Specific examples are cyclohexyl methacrylates, isobornyl acrylate,isobornyl methacrylates and 3,5-dimethyladamantyl acrylate.

Great preference is given to using, for a2), monomers which containhydroxyl groups, very preferably primary hydroxyl groups. Examples ofa2) are hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropylacrylate, hydroxypropyl methacrylate, 6-hydroxyhexyl methacrylate,4-hydroxystyrene, and ally alcohol, this recitation not beingconclusive.

Monomers for a3) are, for example, olefinically unsaturated monomershaving functional groups such as carboxylic acid groups, acid anhydridegroups, phosphonic acid groups, amide or imide or amino groups,isocyanate groups, epoxy groups or thiol groups. Examples of a3) areacrylic acid or methacrylic acid, maleic anhydride, itaconic anhydride,itaconic acid, glyceridyl methacrylate, glyceryl methacrylate, vinylacetic acid, β-acryloyloxypropionic acid, trichloroacrylic acid, fumaricacid, crotonic acid, aconitic acid, acrylonitrile dimethylacrylic acid,N,N-dialkyl-substituted amides, such as N,N-dimethylacrylamide,N,N-dimethylmethacrylamide, N-tert-butylacrylamide, N-vinylpyrrolidone,N-vinyllactam, dimethylaminoethyl methacrylate, dimethylaminoethylacrylate, diethylaminoethyl methacrylate, diethylaminoethyl acrylate,N-methylol-methacrylamide, N-(buthoxymethyl)methacrylamide,N-methylolacrylamide, N-(ethoxy-methyl)acrylamide, andN-isopropylacrylamide, this recitation not being conclusive.

As further hardening comonomers it is possible for macromonomers to havebeen copolymerized into the polyacrylate. Particularly suitablemacromonomers are those as described in EP 1 361 260 A1, such as2-polystyreneethyl methacrylate having a molecular weight Mw of 13 000g/mol. The macromonomer-modified, thermally crosslinked acrylatehotmelts which result on crosslinking have a greater shear strength,owing to the fact that they are physically and thermally crosslinked.

The polyacrylates are particularly suitable for inventive furtherprocessing as carrier material if they are prepared by bulk, solution oremulsion polymerization and if desired are subsequently concentrated,particularly if they contain volatile constituents. Concentration may becarried out advantageously in a devolatizing extruder, particularly inthe same way as DE 102 21 402 A1, page 3, lines 22-68.

In one preferred procedure the polyacrylates have a weight-averagemolecular weight M_(w) of at least 300 000 g/mol up to a maximum of 1500 000 g/mol. The average molecular weight is determined by sizeexclusion chromatography (GPC) or matrix-assisted laserdesorption/ionization coupled with mass spectrometry (MALDI-MS). It canbe advantageous to carry out the polymerization in the presence ofregulators such as thiols, halogen compounds, and, in particular,alcohols (isopropanol), in order to set the desired weight-averagemolecular weight M. The polymerization time, depending on conversion andtemperature, is between 2 and 72 hours.

Also particularly suitable for the inventive further processing arepolyacrylates which have a narrow molecular weight distribution(polydispersity <4). These compositions have a particularly high shearstrength after crosslinking for a relatively low molecular weight. Giventhat, in comparison to a normally distributed polyacrylate, anarrow-distribution polyacrylate needs a lower molecular weight for thesame level of cohesion, there are reductions in viscosity and inoperating temperatures. Hence a narrow-distribution polyacrylate allowsa particularly long processing time.

Narrow-distribution polyacrylates can be prepared by anionicpolymerization or by controlled free-radical polymerization methods, thelatter being especially suitable. Examples are described in U.S. Pat.No. 6,765,078 B2 and DE 10036901 A1 or US 2004/0092685 A1. Atom transferradical polymerization (ATRP) as well can be used with advantage tosynthesize narrow-distribution polyacrylates, the initiator usedpreferably comprising monofunctional or difunctional secondary ortertiary halides and the halide or halides being abstracted usingcomplexes of Cu, Ni, Fe, Pd, Pt, Ru, Os, Rh, Co, Ir, Ag or Au (EP 0 824111 A1; EP 826 698 A1; EP 824 110 A1; EP 841 346 A1; EP 850 957 A1). Thevarious possibilities of ATRP are further described in specificationsU.S. Pat. No. 5,945,491 A, U.S. Pat. No. 5,854,364 A, and U.S. Pat. No.5,789,487 A.

Optionally it is also possible to add plasticizers, resins and fillersto the viscoelastic acrylate layer. Suitable fillers are hydrophilic orhydrophobic silica gels such as Aerosils or Ultrasils, inorganic fillerssuch as chalk, titanium dioxide, calcium sulfate and barium sulfate, andorganic fillers such as polymer beads or fibers based on cellulose,polyethylene, polypropylene, polyamide, polyacrylonitrile, polyester,polymethacrylate and/or polyacrylate.

In addition it is possible for fillers of low flammability, such asammonium polyphosphate, for example, and also electrically conductivefillers, such as conductive carbon black, carbon fibers and/orsilver-coated beads, for example, and also ferromagnetic additives, suchas iron(III) oxides, for example, and also additives for producingfoamed layers, such as expandants, for example, solid glass balls,hollow glass balls, expandable microballoons, aging inhibitors, lightstabilizers and/or ozone protectants to be added or incorporated bycompounding into the polyacrylate before or after the latter has beenconcentrated.

Optionally it is possible to add the typical plasticizers inconcentrations up to 3% by weight. Examples of plasticizers which can bemetered in include low molecular mass polyacrylates, phthalates,water-soluble plasticizers, plasticizer resins, phosphates orpolyphosphates.

The additives can be added before or after the polyacrylate has beenconcentrated.

To produce thick carrier layers it is possible for these layers to beadditionally filled and/or foamed. For these purposes the polyacrylateis admixed with solid glass balls, hollow glass balls or expandingmicroballoons, preferably before the thermal crosslinker is added.

In accordance with the invention a thermal crosslinker is added to thepolyacrylate. In one very advantageous embodiment the added thermalcrosslinker is an isocyanate, preferably a trimerized isocyanate. Withparticular preference the trimerized isocyanates are aliphatic oramine-deactivated isocyanates.

Suitable isocyanates are, in particular, trimerized derivatives of MDI[4,4-methylenedi(phenyl isocyanate)], HDI [hexamethylene diisocyanates,1,6-hexylene diisocyanate] and/or IPDI [isophorone diisocyanates,5-isocyanato-1-isocyanatomethyl-1,3,3-trimethylcyclohexane], examplesbeing the products Desmodur® N3600 and XP2410 (each from BAYER AG:aliphatic polyisocyanates, low-viscosity HDI trimers). Also highlysuitable is the surface-deactivated dispersion of micronized trimerizedIPDI that is BUEJ 339®, now HF9® (BAYER AG).

Also suitable in principle for crosslinking, however, are otherisocyanates, such as Desmodur VL 50 (MDI-based polyisocyanate, BayerAG), Basonat F200WD (aliphatic polyisocyanate, BASF AG), Basonat HW100(water-emulsifiable polyfunctional isocyanate based on HDI, BASF AG),Basonat HA 300 (allophanate-modified polyisocyanate on isocyanurate. HDIbasis, BASF) or Bayhydur VPLS2150/1 (hydrophilically modified IPDI,Bayer AG), this recitation not being conclusive.

The addition of the thermal crosslinker to the polyacrylate takes placein the melt, preferably under precise temperature and time control.

The addition and incorporation of the thermally reactive crosslinkingsystem into the polyacrylate matrix takes place preferably in continuouscompounding assemblies. In accordance with the invention theseassemblies are designed so that, with thorough commixing andsimultaneously low introduction of shearing energy, a short residencetime is ensured for the composition after the crosslinking system hasbeen metered. The compounding assemblies are preferably extruders,especially twin-screw extruders and/or planetary roller extruders. It isparticularly advantageous if the spindles of the extruder are heatableand/or coolable.

With advantage, and in order to optimize the time window of thereaction, the addition may be made in the extruder in which thepolyacrylate composition has already been concentrated.

The crosslinkers are added at one or more locations in the assemblies,preferably in unpressurized zones. It is also favorable if the thermallyreactive crosslinker substances are added in finely divided form to thepolyacrylate, such as in the form of aerosol, in fine droplets, or indilution in a suitable diluent such as a polymer-compatible plasticizer.

With advantage the residual monomer content of the polyacrylate for thecarrier layer when the thermal crosslinker is added is not more than 1%by weight, in particular not more than 0.3% by weight, based on thepolyacrylate.

With advantage the residual solvent content of the polyacrylate for thecarrier layer after concentration when the thermal crosslinker is addedis not more than 1% by weight, in particular not more than 0.3% byweight, based on the polyacrylate.

A preferred procedure is to use the thermal crosslinker, in particularthe trimerized isocyanate, at 0.1% to 5% by weight, in particular at0.2% to 1% by weight, based on the polyacrylate.

In one advantageous development of the method of the invention thetemperature of the polyacrylate for the carrier layer when the thermalcrosslinker is added is between 60° C. and 120° C., more preferablybetween 70° C. and 100° C.

The polyacrylate melt with the crosslinker added is transported to acoating unit, preferably directly through the extruder in which theaddition of the crosslinker and, where appropriate, the concentration ofthe composition have already taken place. There, pretreated, especiallycorona-pretreated, pressure-sensitive adhesive layers are laminated ontoboth sides of the partly crosslinked—that is, not yet fullycrosslinked—viscoelastic carrier layer. Reactive isocyanate groups inthe carrier layer may react at this stage with the hydroxylfunctionalities of the PSA layers laminated on. This has a beneficialeffect on crosslinking. Even compositions which are otherwise verydifficult to anchor can be attached in this way to the viscoelasticcarrier layer.

Suitable adhesive layers of the adhesive tape of the invention comprisein principle all kinds of adhesives that can be anchored on the carrier.

With great preference in a sense of the invention use is made ofpressure-sensitive adhesives (PSAs). In one preferred procedure apretreated, especially corona-pretreated, polyacrylate layer islaminated on to both sides. It is also possible to use adhesive layers,particularly acrylate-based adhesive layers, that have been pretreatedby other methods, provided that in these adhesives there are alreadyfunctional groups, and/or their functional groups formed as a result ofthe pretreatment, that enter into chemical bonds with isocyanate groups.

Particularly preferred PSAs are all-acrylate compositions or those whichcontain no migratable components such as resins or plasticizers in anynotable concentration.

Great preference is given in the sense of the invention to all-acrylatePSAs which have been coated beforehand onto siliconized polyesterliners.

As well as PSAs, suitability is also possessed by hotmelt adhesivelayers or heat-activatable layers, which are laminated preferably ontoone side of the viscoelastic layer. This gives products having aasymmetric construction.

Examples of suitable base polymers for adhesives (both PSAs and otheradhesives) are natural rubber, synthetic rubbers, acrylate blockcopolymers, styrene block copolymers, EVA, many polyolefins,polyurethanes, polyvinyl ethers, and silicones.

Preference is given to adhesives with no notable fractions of migratableconstituents, and which are so highly compatible with the polyacrylateof the carrier layer that they could migrate into it in a significantamount.

Examples of PSA layers very suitable in accordance with the inventionare described in EP 1 308 492 A (page 27, line 23 to page 12, line 56).

The adhesives, preferably PSAs, coated onto release material can bepretreated by all known physical or chemical methods which form reactivegroups that are able to react with isocyanates. It is also possible touse all known techniques for priming, it being possible for the primerlayers to be applied both from solutions or from dispersions onto theadhesive layers. Application may also take place in an extrusion orcoextrusion process. Preferred physical methods are flame pretreatment,corona pretreatment, atmospheric plasma pretreatment or vacuum plasmatreatment. Great preference attaches to corona pretreatment directlybefore the lamination of the still-reactive viscoelastic carrier layer.

In certain cases, particularly when using PSAs with migratableconstituents such as styrene block copolymers or rubber, it may besensible to use a barrier layer, made of polyamide, for example. Othermaterials too, however, which fulfill this purpose are allowable. Then,advantageously, in order to produce a double-sided adhesive assemblytape, either the barrier layer is applied to an adhesive which has beencoated beforehand onto a release material, after which pretreatmenttakes place by the techniques described above on the barrier layer side,and lamination takes place to the still-reactive viscoelastic carrierlayer. Alternatively the barrier layer can be applied to theviscoelastic carrier layer and then coated with the adhesive.

Directly after the operation of coating by means of roll application orextrusion die, the viscoelastic carrier layer is partially crosslinked,but not yet sufficiently crosslinked. Coating in this context means theshaping of the very substantially solvent-free carrier composition,blended with crosslinker, into corresponding layers, and applicationbetween the pretreated adhesive layers applied onto material in webform. The degree of crosslinking at this point in time, therefore, maynot have advanced past a point where the processability of the PSA, inparticular in respect of a good coating pattern, is no longer ensured.

The processing time is 3-30 minutes, preferably 5-20 minutes, morepreferably 5-10 minutes.

The construction of the three-layer adhesive-polyacrylatecarrier-adhesive system takes place preferably via a two-roll unit (cf.FIG. 2). By means of a distributor nozzle (1) or other suitable unit,the viscoelastic composition (3) that forms the eventual carriermaterial, and which is already compounded with the crosslinker and,where appropriate, with fillers, is supplied to the roll nip where it isintroduced between the two adhesive layers (6 a, 6 b), which have beensubjected beforehand to a pretreatment, in particular a corona treatment(8) (corona conduction preferably set: 10 to 200 W min/m²), preferably30-160 W min/m², very preferably 80-120 W min/m². The adhesives, coatedin particular onto antiadhesive auxiliary carriers (7 a, 7 b), areintroduced into the apparatus via the rolls W1 and W2, in such a waythat the adhesive sides are facing one another.

The shaping of the viscoelastic composition into a viscoelastic filmtakes place between the calender rolls W1 and W2 in the roll nip, and atthe same time there is coating with the two supplied adhesives (6 a, 6b). The purpose of pretreating the adhesives, in particular within acorona station (8), is to improve the anchoring of the adhesives on theshaped, viscoelastic carrier layer. This treatment produces active OHgroups on the surface of the adhesives, which after the three-layerassembly has been produced lead to improved chemical attachment to theviscoelastic carrier layer.

The width of the roll nip and the pressure of the rolls determine thelayer thickness of the carrier.

The process outlined above is particularly suitable for producingviscoelastic three-layer constructions with layer thicknesses between100 μm and 10 000 μm, preferably between 300 μm and 5000 μm, atproduction speeds of between 0.5 m/min and 100 m/min.

Depending on the viscoelastic compositions and antiadhesive carriermaterials used, the surface temperatures of the rolls are set atpreferably between 25° C. and 200° C., more preferably between 60° C.and 150° C., and very preferably between 80° C. and 120° C. Suitablesurfaces for the two calender rolls used include all of the materialsthat are familiar to the skilled worker, such as steel, chromium-platedsteel, stainless steel, plastics, silicones, ceramics, and combinationsof the stated materials.

Amazingly, and surprisingly for the skilled worker, the bead ofcomposition rotating within the roll nip does not undergo mixing withthe supplied adhesives. It would have been expected that the adhesivewould be dissolved at least partly by the antiadhesively furnishedcarrier, and would undergo mixing with the rotating bead of composition.

The crosslinking reaction, especially with isocyanates, proceedspreferably without catalyses. In the case of functionalized acrylatecopolymers that contain no copolymerized acrylic acid, the reactionproceeds preferably with aromatic and/or aliphatic isocyanates atslightly elevated temperatures. In the case of functionalized acrylatecopolymers that contain copolymerized acrylic acid, the reaction rate isfaster. Here an operationally stable process is accomplished preferablywith the slower-reacting aliphatic isocyanates or surface-deactivatedisocyanate emulsions.

Even without any heat being supplied, the crosslinking reaction proceedsto completion under standard conditions (room temperature). Generally,the crosslinking reaction with the multifunctionalized isocyanate isvery largely at an end after a storage period of up to 14 days, inparticular of four to ten days, and the ultimate cohesion of thecomposition has been reached. At the same time, as a result, theviscoelastic carrier layer attaches chemically to the outer layers ofadhesive that have been laminated on.

The crosslinking with isocyanates forms urethane groups which link thepolymer chains. This linkage results in an increase in the cohesion ofthe viscoelastic carrier layer and hence in the shear strength of theproduct.

The physical properties of the viscoelastic carrier and of the endproduct, particularly the viscosity, flow-on behavior, heat stability,and shear strength, can be influenced not only by the comonomers and,where appropriate, fillers selected but also by the degree ofcrosslinking, thereby allowing the end product to be optimized throughan appropriate choice of the reaction conditions. Various factorsdetermine the operational window of this process. The most importantinfluencing variables, particularly in the case of unblended systems,are operational temperature and coating temperature, residence time inthe compounding extruder and coating assembly, type of crosslinker(deactivated, aliphatic, aromatic), crosslinker concentration, fractionof hydroxyl groups in the polymer, fraction of copolymerized acid groupsin the polymer, and the average molecular weight of the polyacrylate.

A number of relations are described below with regard to the preparationof the polyacrylate of the invention, these relations optimizing theproduction method but not being restrictive of the concept of theinvention:

For a given concentration of crosslinker, an increase in the operatingtemperature leads to a reduced viscosity, which enhances the coatabilityof the viscoelastic composition but reduces the processing time. Anincrease in processing time is obtained by lowering the crosslinkerconcentration, lowering the molecular weight, lowering the concentrationof hydroxyl groups in the polymer, lowering the acid fraction in thepolymer, using less reactive isocyanates, and lowering the operatingtemperature. An improvement in the cohesion of the viscoelasticcomposition can be obtained in different ways. One way is to raise thecrosslinker concentration, which reduces the processing time. With thecrosslinker concentration constant, it is also possible to raise themolecular weight of the polyacrylate, which is possibly more efficient.The abovementioned parameters must be adapted appropriately inaccordance with the desired profile of requirements of the compositionand/or the product.

Where expanding microballoons are used in the production of foamedlayers, the composition or the shaped layer is advantageouslyactivatable in an appropriate manner by introduction of heat. Acorona-pretreated polyacrylate layer can then be laminated on,preferably, to both sides of this foamed layer. Advantageously thefoamed layer is smoothed by the lamination of the corona-pretreated PSAlayers, or separately, by means of rolls, for example.

As a result of the combination of the carrier formulas and adhesiveformulas, the adhesive tape of the invention exhibits outstandingproperties, of a kind which could not have been foreseen by the skilledworker, and, consequently, the use of the tape is possible in particularas an adhesive assembly tape for high-performance applications. This isalso true in particular for its implementation as a self-adhesive tape.The inventive advantages are significant in particular at applicationtemperatures of between 100 and 130° C. Furthermore, on account of thevery good anchoring and homogeneous crosslinking, the plasticizerresistance is outstanding. The bond strength on plasticizer-containingsubstrates remains high even after storage, and the anchoring of thelayers to one another remains very good. Delamination of the layers isnot observed even following storage against plasticizer-containingsubstrates.

On account of the high flexibility of the carrier, the adhesive tapeconforms very well to uneven substrates. A durable bond is producedbetween adhesive tape and substrate, and does not fail even under highshearing forces and bending-moment stresses, even at high temperaturesand even after storage under UV irradiation and moisture.

An adhesive tape of this kind can be used, for example, in the furnitureindustry, where mirrors, strips or trim are to be durably bonded. Onaccount of the outstanding properties of the product, its use as anassembly material is also advantageous in many areas of industry, whendifferent surfaces, especially UV-transparent surfaces, such as windowglass or transparent plastics, are to be durably bonded to one another.

As a result of the homogeneous crosslinking through the layer, thetechnical adhesive performance is equally good on each side. With thesame adjacent adhesive layers and the same surface structures, thetechnical adhesive properties of the respective sides are also the same.The viscoelastic carriers do not exhibit any profile of crosslinkingthrough the layer.

With the method of the invention it is possible, furthermore, to offervery thick adhesive tapes. The viscoelastic carrier layer can beparticularly thick, since in principle this process is not subject toany limit on layer thickness, in contrast to crosslinking via UV or EBCirradiation. In particular it is also possible to produce filled andfoamed viscoelastic layers. In addition, prior to the addition of thethermal crosslinker, it is sensible to add solid glass beads, hollowglass beads or expanding microballoons to the polyacrylate. Whereexpanding microballoons are used, the composition or the shaped layer ispreferably activated in an appropriate way by means of introduction ofheat. A corona-pretreated polyacrylate layer can then be laminated on,preferably, on both sides to this foamed layer. Advantageously thefoamed layer is smoothed by the lamination of the corona-pretreatedadhesive layers or separately, between two rollers, for example.

It is possible to obtain layers colored and filled in any desired way.Moreover, the residual monomer content of the viscoelastic layers, as ofthe three-layer product as a whole, is very low. A particularlyadvantageous possibility is that of being able to produce thehomogeneously crosslinked viscoelastic acrylate carrier of thethree-layer, double-sidedly adhesive-furnished adhesive tape at coatingspeeds of above 50, preferably 100 m/min.

For certain applications the adhesive tape of the invention, in thiscase in the form of an intermediate product, can be improved or furtheradapted to requirements by means of additional irradiation with actinicradiation (UV light or electron beams, for example).

EXAMPLES

The exemplary embodiments which follow are intended to illustrate theinvention, without the choice of the examples given being intended torestrict the invention unnecessarily.

Test Methods:

Solids Content:

The solids content is a measure of the fraction of nonvolatiles in apolymer solution. It is determined gravimetrically by weighing thesolution, then evaporating the volatile fractions in a drying cabinet at120° C. for 2 hours, and weighing the residue again.

K Value (According to FIKENTSCHER):

The K value is a measure of the average size of molecules ofhigh-polymer compounds. It is measured by preparing one percent (1 g to100 ml) toluene solutions of polymer and determining their kinematicviscosities using a VOGEL-OSSAG viscometer. Standardizing to theviscosity of the toluene gives the relative viscosity, from which the Kvalue can be calculated by the method of FIKENTSCHER (Polymer 8/1967,381 ff.).

Gel Permeation Chromatography GPC

The average molecular weight M_(w) and the polydispersity PD weredetermined by the company Polymer Standards Service at Mainz. The eluentused was THF containing 0.1% by volume trifluoroacetic acid. Measurementwas made at 25° C. The precolumn used was of type PSS-SDV, 5μ, 10³ Å, ID8.00 mm×50 mm. Separation was carried out using the columns of typePSS-SDV, 5μ, 10³ and also 10⁵ and 10⁶ each of ID 8.0 mm×300 mm. Thesample concentration was 4 g/l and the flow rate 1.0 ml per minute.Measurement was made against PMMA standards.

90° Bond Strength to Steel (Open and Lined Sides)

The bond strength to steel is determined under test conditions of23°+/−1° C. room temperature and 50%+/−5% relative atmospheric humidity.The specimens were cut to a width of 20 mm and adhered to a steel plate.Prior to the measurement, the steel plate must be cleaned andconditioned. For that purpose the plate is first wiped with acetone andthen left in the air for 5 minutes to allow the solvent to evaporate.The side of the transfer tape facing away from the test substrate wasthen lined with a 50 μm aluminum foil, to prevent the specimenstretching in the course of measurement. After that, the test specimenwas rolled onto the steel substrate. For that purpose the tape was runover back and forth with a 2 kg roller 5 times, at a rolling speed of 10m/min. Immediately after this rolling, the steel plate was inserted intoa special mount which allows the specimen to be peeled vertically upwardat an angle of 90°. The bond strength was measured using a Zwick tensiletesting machine. In the case of application of the lined side to thesteel plate, the open side of the transfer tape is first laminatedagainst the 50 μm aluminum foil, the release material is removed and thetape is adhered to the steel plate, rolled on in the same way andsubjected to measurement.

The results measured for both sides, open and lined, are reported inN/cm and have been averaged from three measurements.

Holding Power (Open and Lined Sides)

Sample preparation took place under test conditions of 23° C.+/−1° C.room temperature and 50%+/−5% relative atmospheric humidity. The testspecimen was cut to 13 mm and adhered to a steel plate. The bond area is20 mm×13 mm (length×width). Prior to the measurement, the steel platewas cleaned and conditioned. For that purpose the plate is first wipedwith acetone and then left in the air for 5 minutes to allow the solventto evaporate. After bonding, the open side was reinforced with a 50 μmaluminum foil and rolled over twice back and forth with a 2 kg roller.Then a belt loop was attached to the protruding end of the transfertape. The whole assembly was then suspended from a suitable apparatusand loaded with 10 N. The suspension apparatus is such that the weightloads the sample at an angle of 179°+/−1°. This ensures that thetransfer tape cannot peel from the bottom edge of the plate. Themeasured shear withstand time, the time between suspension of thespecimen and its falling off, is reported as holding power in minutesand corresponds to the average value from three measurements. For themeasurement of the lined side, the open side is first reinforced withthe 50 μm aluminum foil, the release material is removed, and the tapeis adhered to the test plate in the same way as described. Themeasurement is made under standard conditions (23° C., 55% atmospherichumidity).

Rolling Ball Tack (Open and Lined Sides)

The rolling ball test was used to measure the tack of the specimens witha very short contact time. Measurement took place under test conditionsof 23° C.+/−1° C. room temperature and 50%+/−5% relative atmospherichumidity. The transfer tape was fixed, with the side under test facingupward, under gentle tension, on the working plate, which was orientedexactly horizontally. Subsequently a ramp 65 mm high was placed on thespecimen strip and a clean ball, cleaned with acetone and weighing 5.6g, was rolled down the ramp. The distance between the leading edge ofthe ramp and the center point of the rolled ball which has come to restwas measured. The value reported is the average from 5 measurements perside.

SAFT—Shear Adhesive Failure Temperature (Open and Lined Sides)

The SAFT test is an accelerated test of the short-term temperatureresistance of the transfer tapes. The specimens were reinforced with a50 μm aluminum foil and the remaining adhesive side was adhered to aground steel test plate which had been cleaned with acetone, and thenoverrolled six times using a 2 kg steel roller at a speed of 10 m/min.The bond area of the sample, height×width, was 13 mm×10 mm. The top partof the specimen, which protrudes beyond the test plate by 2 mm, wasreinforced with a shear adhesive strip. At this point, after the samplehad been suspended vertically, the travel sensor was applied.

The sample under measurement was loaded at the bottom end with a weightof 50 g. The steel test plate with the bonded sample was then heated,starting at 25° C. and at a rate of 9° C. per minute, to the finaltemperature of 200° C. Using the travel sensor, the slip travel of thesample was measured as a function of temperature and time. Themeasurement was ended when the envisaged final temperature was reachedor when a slip travel of >1000 μm was attained.

The SAFT test is able to provide two test features: SAFT shear travel orSAFT short-term temperature resistance. The SAFT shear travel is theslip travel in μm when the final temperature is reached. The SAFTshort-term temperature resistance is the temperature at which a sliptravel of 1000 μm is attained. Both sides are measured: the open sideand the lined side. The value reported is in each case the average of aduplicate determination.

Wall Hook Test

FIG. 3 shows the testing of the pressure-sensitive polyacrylate layers(layer A and/or C). A test specimen (3.1) fixed between two polishedsteel plates (3.2) and measuring 30 mm×30 mm is pressed for 1 minute at0.9 kN (force P). After that, a lever arm (3.3) 9 cm long is screwedinto the top steel plate, which is subsequently loaded with a 1000 gweight (3.4). Care is taken to ensure that the time between pressing andloading is not more than 2 minutes (t≦2 min).

A measurement is made of the holding power, i.e., the time between thesuspension of the specimen and its falling off. The result reported isthe holding power in minutes, as the average value from a triplicatedetermination. The test conditions are 23° C.+/−1° C. and 50% rh+/−5% rh(rh=relative humidity).

Measurements were carried out in each case on the open side and thelined side.

Pressure-Sensitive Polyacrylate Adhesive 1 (PA1):

A 100 l glass reactor conventional for free-radical polymerizations wascharged with 2.8 kg of acrylic acid, 8.0 kg of methyl acrylate, 29.2 kgof 2-ethylhexyl acrylate and 20.0 kg of acetone/isopropanol (95:5).After nitrogen gas had been passed through the reactor for 45 minuteswith stirring, the reactor was heated to 58° C. and 20 g ofazoisobutyronitrile (AIBN, Vazo 64®, DuPont) were added. Subsequentlythe external heating bath was heated to 75° C. and the reaction wascarried out constantly at this external temperature. After a reactiontime of 1 h a further 20 g of AIBN were added. After 4 h and 8 h thereaction mixture was diluted with 10.0 kg each time ofacetone/isopropanol (95:5) mixture. For reduction of the residualinitiators, after both 8 h and 10 h, 60 g portions ofbis(4-tert-butylcyclohexanyl) peroxydicarbonate (Perkadox 16®, AkzoNobel) were added. The reaction was discontinued after a reaction timeof 24 h and the reaction mixture was cooled to room temperature.Subsequently the polyacrylate was blended with 0.4% by weight ofaluminum(III) acetylacetonate (3% strength solution, isopropanol), theblend was diluted with isopropanol to a solid contents of 30%, and thencoating took place from solution onto a siliconized release film (50 μmpolyester). (Coating speed 2.5 m/min, drying tunnel 15 m, temperatures:zone 1: 40° C., zone 2: 70° C., zone 3: 95° C., zone 4: 105° C.). Thecoat weight was 50 g/m².

Pressure-Sensitive Polyacrylate Adhesive 2 (PA2):

A 100 l steel reactor conventional for free-radical polymerizations wascharged with 4.0 kg of acrylic acid, 36.0 kg of 2-ethylhexyl acrylateand 13.3 kg of acetone/isopropanol (96:4). After nitrogen gas had beenpassed through the reactor for 45 minutes with stirring, the reactor washeated to 58° C. and 20 g of azoisobutyronitrile (AIBN, Vazo 64®,DuPont) were added. Subsequently the external heating bath was heated to75° C. and the reaction was carried out constantly at this externaltemperature. After a reaction time of 1 h a further 20 g of AIBN wereadded. After 4 h and 8 h the reaction mixture was diluted with 10.0 kgeach time of acetone/isopropanol (96:4) mixture. For reduction of theresidual initiators, after both 8 h and 10 h, 60 g portions ofbis(4-tert-butylcyclohexanyl)peroxydicarbonate (Perkadox 16®, AkzoNobel) were added. The reaction was discontinued after a reaction timeof 24 h and the reaction mixture was cooled to room temperature.Subsequently the polyacrylate was blended with 0.4% by weight ofaluminum(III) acetylacetonate (3% strength solution, isopropanol), theblend was diluted with isopropanol to a solids content of 30%, and thencoating took place from solution onto a siliconized release film (50 μmpolyester). After drying (coating speed 2.5 m/min, drying tunnel 15 m,temperatures: zone 1: 40° C., zone 2: 70° C., zone 3: 95° C., zone 4:105° C.) the coat weight was 50 g/m².

B. Production of the Viscoelastic Carriers

Preparation of the Starting Polymers for the Viscoelastic Carriers ofExamples VT 1 to 6

The preparation of the starting polymers is described below. Thepolymers investigated were prepared conventionally via free-radicalpolymerization in solution.

HEMA=hydroxyethyl methacrylate

AIBN=2,2′-azobis(2-methylbutyronitrile)

Perkadox 16=bis(4-tert-butylcyclohexyl)peroxydicarbonate

Base Polymer 1 (B1)

A reactor conventional for free-radical polymerizations was charged with27 kg of 2-ethylhexyl acrylate, 27 kg of n-butyl acrylate, 4.8 kg ofmethyl acrylate, 0.6 kg of acrylic acid, 0.6 kg of HEMA and 40 kg ofacetone/isopropanol (93:7). After nitrogen gas had been passed throughthe reactor for 45 minutes with stirring, the reactor was heated to 58°C. and 30 g of AIBN were added. Subsequently the external heating bathwas heated to 75° C. and the reaction was carried out constantly at thisexternal temperature. After 1 h a further 30 g of AIBN were added andafter 4 h the batch was diluted with 10 kg of acetone/isopropanolmixture.

After 5 h and after 7 h, reinitiation was carried out with 90 g ofPerkadox 16 each time. After a reaction time of 22 h the polymerizationwas discontinued and the product was cooled to room temperature. Thepolyacrylate has a K value of 69, a solids content of 54.6%, an averagemolecular weight of Mw=819 000 g/mol, and a polydispersity (Mw/Mn)=7.6.

Base Polymer 2 (B2)

In the same way as for example 1, 36.0 kg of 2-ethylhexyl acrylate, 21.0kg of tert-butyl acrylate, 2.4 kg of acrylic acid and 0.6 kg of HEMAwere polymerized in 40 kg of acetone/isopropanol (93:7). Initiation wascarried out twice with 30 g of AIBN each time, twice with 90 g ofPerkadox 16 each time, and dilution was carried out with 10 kg ofacetone/isopropanol mixture (93:7). After a reaction time of 22 h thepolymerization was discontinued and the product was cooled to roomtemperature.

The polyacrylate has a K value of 60.0, a solids content of 53.5%, anaverage molecular weight of Mw=602 000 g/mol, and a polydispersity(Mw/Mn)=7.1.

Base Polymer 3 (B3)

The same procedure was used as for example 1. For the polymerization, 36kg of 2-ethylhexyl acrylate, 20.4 kg of methyl acrylate, 2.4 kg ofacrylic acid and 1.2 kg of HEMA were polymerized in 40 kg ofacetone/isopropanol (90:10). Initiation was carried out twice with 30 gof AIBN each time, twice with 90 g of Perkadox 16 each time, anddilution was carried out with 10 kg of acetone/isopropanol mixture(90:10). After a reaction time of 22 h the polymerization wasdiscontinued and the product was cooled to room temperature.

The polyacrylate has a K value of 57, a solids content of 53.8%, anaverage molecular weight of Mw=526 000 g/mol, and a polydispersity(Mw/Mn)=6.8.

Base Polymer 4 (B4)

A reactor conventional for free-radical polymerizations was charged with36 kg of 2-ethylhexyl acrylate, 21 kg of tert-butyl acrylate, 2.4 kg ofacrylic acid, 0.6 kg of HEMA, 40 g of benzyl dithiobenzoate and 40 kg ofacetone. After nitrogen gas had been passed through the reactor for 45minutes with stirring, the reactor was heated to 58° C. and 30 g of AIBNwere added. Subsequently the external heating bath was heated to 75° C.and the reaction was carried out constantly at this externaltemperature. After 1 h a further 30 kg of AIBN were added and after 4 hthe batch was diluted with 5 kg of acetone. After 5 h and after 7 h, 90g portions of Perkadox 16™ (Akzo) were added. After a reaction time of22 h the polymerization was discontinued and the product was cooled toroom temperature. The polyacrylate has a K value of 53.6, a solidscontent of 54.9%, an average molecular weight of Mw=479 000 g/mol, and apolydispersity (Mw/Mn)=2.4.

Method 1: Concentration of the Base Polymers for the ViscoelasticCarriers:

The acrylate copolymers (base polymer 1-4) functionalized with hydroxylgroups are freed very largely from the solvent by means of a BERSTORFFsingle-screw extruder (concentrating extruder). The parameters given byway of example here are those for the concentration of base polymer 1.The speed of the screw was 160 rpm, the motor current 16 A, and athroughput of 61.5 kg liquid/h was realized. For concentration, a vacuumwas applied at 3 different domes. The reduced pressures were,respectively, 440 mbar, 50 mbar and 5 mbar, the lowest vacuum beingapplied in the first dome. The exit temperature of the concentratedhotmelt was 104° C. The solids content after this concentration step was99.8%.

Method 2: Production of the Filler-Modified Viscoelastic Carriers,Blending with the Thermal Crosslinker

The acrylate polymers concentrated by method 1 were melted in a feederextruder (single-screw conveying extruder from TROESTER) and using thisextruder were conveyed as a polymer melt into a twin-screw extruder(LEISTRITZ, Germany, ref. LSM 30/34). The assembly is heatedelectrically from the outside and is air cooled by a number of fans. Thegeometry of the mixing screws was chosen such that effectivedistribution of the fillers and of the crosslinking system in thepolymer matrix is accompanied by the assurance of a short residence timeof the acrylate composition in the extruder. For these purposes themixing screws of the twin-screw extruder were arranged so that conveyingelements are in alternation with mixing elements. The addition of thefillers and of the respective crosslinking system takes place withappropriate metering equipment, at two or more sites where appropriate,into the unpressurized conveying zones of the twin-screw extruder.Metering aids are used where appropriate to meter the crosslinkingsystem. It is possible if desired to connect a vacuum pump to thetwin-screw extruder in order to free the compounded self-adhesivecomposition from gas inclusions. The ready-compounded acrylatecomposition is then supplied, by means of a melt pump downstream of themixing extruder, to a distributor nozzle, which conveys the viscoelasticcarrier into the first roll nip of the coating calender. Coating of theself-adhesive compositions of the invention takes place by means of atwo-roll calender in accordance with one of the methods described below.

Method 3: Production of the Three-Layer Constructions by Means ofTwo-Roll Calender

The method was carried out as described in FIG. 1. Using distributornozzle (1) the viscoelastic composition (3), already compounded with thecrosslinking system and, where appropriate, fillers, is supplied to theroll nip. The shaping of the viscoelastic composition to a viscoelasticfilm takes place between the calender rolls (W1) and (W2) in the rollnip between two self-adhesive compositions (7 a, 7 b), which in turn aresupplied coated onto antiadhesively furnished carrier materials (5 a, 5b). In this case there is, simultaneously, shaping of the viscoelasticcomposition to the set layer thickness, and coating with the twoself-adhesive compositions supplied. In order to improve the anchoringof the self-adhesive compositions (7 a, 7 b) on the shaped viscoelasticcarrier layer (4), the self-adhesive compositions, before being fed intothe roll nip, are corona-treated by means of corona station (8) (coronaunit from VITAPHONE, Denmark, 100 W min/m²). This treatment producesactive OH groups on the surface of the self-adhesive compositions, whichafter the three-layer assembly has been produced lead to improvedchemical attachment to the viscoelastic carrier layer.

The web speed on passing through the coating unit is 40 m/min.

Downstream of the roll nip, an antiadhesive carrier (5 a) is lined whereappropriate, and the finished three-layer product (6) is wound up withthe remaining second antiadhesive carrier (5 b).

Presented below are specific examples relating to the production of theself-adhesive compositions and coating of the adhesive tapes of theinvention, without any intention that the invention should beunnecessarily restricted by the choice of specified formulations,configurations, and operational parameters.

Example MT 1

The base polymer B1 was concentrated by method 1 (solids content 99.7%)and then blended by method 2 with 1.6% by weight (based on acrylatecopolymer) of the trimerized aliphatic diisocyanates Desnnodur XP 2410(BAYER AG, Germany). To improve its capacity for being metered, thetrimerized diisocyanate was diluted 1:3 with the liquid phosphate esterREOFOS 65 (GREAT LAKES, USA). The operational parameters are summarizedin table 1. Coating between the composition layers PA 1, which have beencoated beforehand onto siliconized polyester films, takes place on thetwo-roll applicator mechanism at roll temperatures of 100° C. by method3. The layer thickness of the viscoelastic carrier VT 1 was 82 μm. Thecorona power was 100 W min/m².

After 7 days' room-temperature storage, the technical adhesive data weremeasured for both the open and the lined sides. The data of example 1are summarized in table 2.

Example MT 2

The base polymer B1 was concentrated by method 1 (solids content 99.7%)and then blended in the same way as in example 1 with 0.8% by weight(based on acrylate copolymer) of the trimerized aliphatic diisocyanatesDesmodur XP 2410 (BAYER AG, Germany). Subsequently, in the same way asin example 1, coating between composition layers PA 1, which have eachbeen coated beforehand onto siliconized polyester films, takes place onthe two-roll applicator mechanism by method 3. The layer thickness ofthe viscoelastic carrier VT 2 was 800 μm. The corona power was 100 Wmin/m².

After 7 days' room-temperature storage, the technical adhesive data weremeasured for both the open and the lined sides. The data of example MT 2are summarized in table 2.

Example MT 3

The base polymer B1 was concentrated by method 1 (solids content 99.7%)and then blended by method 2 with 6.5% by weight of hollow glass beadsQ-CEL 5028 (Potters Industries) and 2.13% by weight (based on acrylatecopolymer) of the hydrophilic aliphatic polyisocyanates Bayhydur VP LS2150/1 (BAYER AG, Germany). The operational parameters are summarized intable 1. Coating between the composition layers PA 1, which have beencoated beforehand onto siliconized polyester films, takes place on thetwo-roll applicator mechanism at roll temperatures of 100° C. by method3. The layer thickness of the viscoelastic carrier VT 3 was 865 μm. Thecorona power was 100 W min/m².

After 7 days' room-temperature storage, the technical adhesive data weremeasured for both the open and the lined sides. The data of example MT 3are summarized in table 2.

Example MT 4

The base polymer B2 was concentrated by method 1 (solids content 99.7%)and then blended by method 2 with 18% by weight of Mikrosohl chalk(MS40, Sohlde) and 0.19% by weight (based on acrylate copolymer) of thehydrophilic aliphatic polyisocyanates Bayhydur VP LS 2150/1 (BAYER AG,Germany). The operational parameters are summarized in table 1. Coatingbetween the composition layers PA 1, which have been coated beforehandonto siliconized polyester films, takes place on the two-roll applicatormechanism at roll temperatures of 100° C. by method 3. The layerthickness of the viscoelastic carrier VT 4 was 790 μm. The corona powerwas 100 W min/m².

After 7 days' room-temperature storage, the technical adhesive data weremeasured for both the open and the lined sides. The data of example MT 4are summarized in table 2.

Example MT 5

The base polymer B3 was concentrated by method 1 (solids content 99.8%)and then blended by method 2 with 3% by weight of nonexpanded hollowmicrobeads Expancel 092 DU 40 (Akzo Nobel, Germany) and 1.0% by weight(based on acrylate copolymer) of the trimerized aliphatic diisocyanatesDesmodur XP 2410 (BAYER AG, Germany). The operational parameters aresummarized in table 1. Heat is introduced to expand the mixture in theextruder, and then coating between the composition layers PA 1, whichhave been coated beforehand onto siliconized polyester films, takesplace at roll temperatures of 130° C. by method 3. The layer thicknessof the expanded viscoelastic carrier VT 5 was 740 μm. The corona powerfor pretreating the pressure-sensitive adhesive layers was 100 W min/m².

After 7 days' room-temperature storage, the technical adhesive data weremeasured for both the open and the lined sides. The data of example MT 5are summarized in table 2.

Example MT 6

The base polymer B6 was concentrated by method 1 (solids content 99.8%)and then blended by method 2 with 5% by weight of hydrophobized silicagel Aerosil R 972 (Degussa, Germany) and 1.0% by weight (based onacrylate copolymer) of the trimerized aliphatic diisocyanates DesmodurXP 2410 (BAYER AG, Germany). The operational parameters are summarizedin table 1. Coating between the composition layers PA 1, which have beencoated beforehand onto siliconized polyester films, subsequently takesplace at roll temperatures of 100° C. by method 3. The layer thicknessof the viscoelastic carrier VT 6 was 750 μm. The corona power forpretreating the pressure-sensitive adhesive layers was 100 W min/m².

After 7 days' room-temperature storage, the technical adhesive data weremeasured for both the open and the lined sides. The data of example MT 6are summarized in table 2.

Example MT 7

In analogy to the production of MT 3, the base polymer B1 wasconcentrated by method 1 (solids content 99.7%), blended by method 2with 6.5% by weight of hollow glass beads Q-CEL 5028 (PottersIndustries) and 2.13% by weight (based on acrylate copolymer) of thehydrophilic aliphatic polyisocyanates Bayhydur VP LS 2150/1 (BAYER AG,Germany), and coated by method 3 at roll temperatures of 100° C. betweenthe composition layers PA 2, which had been coated beforehand ontosiliconized polyester films. The layer thickness of the viscoelasticcarrier VT 3 was 865 μm. The corona power for pretreating thepressure-sensitive adhesive layers was 100 W min/m².

After 7 days' room-temperature storage, the technical adhesive data weremeasured for both the open and the lined sides. The data of example MT 7are summarized in table 2.

Completely surprising for the skilled worker is the coatability of thehotmelt composition following the addition and incorporation by mixingof the isocyanate crosslinking system into the polyacrylate compositionat the temperatures of between 60° C. and 120° C., preferably between70° C. and 100° C., prevailing within the compounding assembly. Bycoatability is meant in this context the capacity for the shaping of thecrosslinker-blended polyacrylate composition into viscoelastic carrierlayers and for its application between corona-pretreatedpressure-sensitive adhesives which are in the form of webs and are onrelease material, this application taking place by means of coatingnozzles or roll coating mechanism.

What was expected was a crosslinking or gelling of the viscoelasticcarrier composition, so that subsequent application by coating would nolonger have been possible. In actual fact, however, the viscoelasticcarrier compositions described can be applied by coating within acertain time period after the metered addition of crosslinker, andcompounding.

The processing time is heavily dependent on molecular weight andhydroxyl functionalization of the polyacrylate composition, and also onthe type and amount of the crosslinking system used and the prevailingoperational conditions, such as composition temperature and geometry ofthe compounding assembly, for example.

In accordance with the known prior art, the skilled worker would haveexpected an immediate reaction of the isocyanates with the OH groupspresent in the polyacrylate, thereby making the partially crosslinkedcomposition undercoatable. To avoid this, he or she would have had touse blocked isocyanates at extremely high temperatures, with thedisadvantage of the blocking agents remaining in the adhesive andcausing disruption. This would severely disrupt the adhesive performanceof the adjacent pressure-sensitive adhesive layers.

Nor would the skilled worker have expected that effectiveaftercrosslinking of the viscoelastic carrier compositions would bepossible at room temperature without the controlled influence of actinicradiation, as shown significantly by SAFT and the 10N steel holdingpower.

As is apparent from the data in the table, the inventivelydouble-sidedly adhesive assembly tapes have very good technical adhesivedata. A particular positive is the balanced adhesive profile of therespective sides. With the same layer of adhesive on both sides of theadhesive tape, their technical adhesive data are virtually the same.This shows the homogeneous crosslinking through the layer. This issurprising for the skilled worker. Moreover, these three-layer adhesivetapes do not exhibit any delamination. The anchoring of the layers toone another is very good, as a result of the corona treatment of thepressure-sensitive adhesive layers and the aftercrosslinking of theadjacent viscoelastic carrier layer.

TABLE 1 Viscoelastic carriers Crosslinker incorporation and coatingFraction of Crosslinker Throughput of Rotational Setpoint Base polymeradjuvants type and amount composition speed temperature K [% by [%crosslinker through TSE of TSE TSE Example Polymer value weight] basedon polymer] [kg/h] [1/min] [° C.] VT 1 B1 69 — 1.6% 10 100 100 XP 2410VT 2 B1 69 — 0.8% 10 100 100 XP 2410 VT 3 B1 69 6.5% 2.13% 11 100 80Hollow Bayhydur glass VP LS beads 2150/1 Q-CEL 5028 VT 4 B2 60 18% 0.19%16 150 100 Mikrosöhl Bayhydur MS 40 VP LS C160 2150/1 VT 5 B3 57 3% 1.0%10 100 125 Expancel XP 2410 092 DU 40 VT 6 B4 54 5% 1.0% 10 100 100Aerosil R XP 2410 972 Crosslinker incorporation and coating CurrentPressure Temperature Coating temperature consump- at exit of compositiondoctor roll/ Processing Coat- Layer tion TSE TSE after TSE coating rolltime weight thickness Example [A] [bar] [° C.] [° C.] [min] [g/m²] [μm]VT 1 12 15 100 100/100 10 870 825 VT 2 11 15 98 100/100 15 840 800 VT 311 15 98 100/100 5 750 865 VT 4 19 38 123 100/100 15 985 790 VT 5 10 28140 130/130 5 375 740 VT 6 18 25 100 100/100 10 800 750

TABLE 2 Product construction and technical adhesive data for thethree-layer constructions Bond strength to steel Holding power 10N Walltest SAFT Three-layer product Total [N/cm] 23° C. [min] [min] [μm]Viscoelastic thickness Open Lined Open Lined Open Lined Open LinedExample PSA 1 carrier layer PSA 2 [μm] side side side side side sideside side MT 1 50 g/m² VT 1 50 g/m² 923 10.6 10.7   7300   7368 54005300 750 μm 754 μm PA 1 PA 1 (200° C.) (200° C.) MT 2 50 g/m² VT 2 50g/m² 900 11.8 11.6   5328   5450 3876 3860 937 μm 915 μm PA 1 PA 1 (200°C.) (200° C.) MT 3 50 g/m² VT 3 50 g/m² 862 8.5 8.7 >20 000 >20 000 93209360 340 μm 325 μm PA 1 PA 1 (200° C.) (200° C.) MT 4 50 g/m² VT 4 50g/m² 885 10.7 10.6  15 460  15 386 7540 7468 545 μm 552 μm PA 1 PA 1(200° C.) (200° C.) MT 5 50 g/m² VT 5 50 g/m² 838 13.5 13.6 >20 000 >20000 >10 000   >10 000   795 μm 801 μm PA 1 PA 1 (200° C.) (200° C.) MT 650 g/m² VT 6 50 g/m² 850 12.7 12.6 >20 000 >20 000 >10 000   >10 000  250 μm 256 μm PA 1 PA 1 (200° C.) (200° C.) MT 7 50 g/m² VT 3 50 g/m²964 9.5 9.4 >20 000 >20 000 >10 000   >10 000   322 μm 342 μm PA 2 PA 2(200° C.) (200° C.)

1-13. (canceled)
 14. An adhesive tape having at least one carrier layerand two outer adhesive layers, wherein the carrier layer is composed ofa photoinitiator-free homogeneously crosslinked polymer of acrylateand/or methacrylate, and wherein the thickness of the adhesive tape isbetween 300 and 10000 μm.
 15. The adhesive tape of claim 1, wherein atleast one of the adhesive layers are formed of pressure-sensitiveself-adhesive compositions.
 16. The adhesive tape of claim 2, whereinboth adhesive layers are formed of pressure-sensitive self-adhesivecompositions.